2023 EXAMINATIONS
MSc
ENGINEERING
(2 hours)
ENGR527: High Frequency Electronics
Answer ALL questions
Appendices Included:
Smith Charts
2
A1 Answer ALL parts (a) – (d)
(a) A 50 Ohm RF source is connected to a terminated line and the reflection
coefficient is measured to be S11=0.5+j0.2. If the line has an electrical length
of 90 degrees, what is the impedance of the load at the end of the line?
(8 marks)
(b) Using the Smith chart provided, design an LC matching section to be placed
between the source and the line at 500 MHz. Please show the paths on the
Smith chart each component provides, as well your calculations of the
inductance and capacitance of each component, and the circuit diagram.
(18 marks)
(c) What is the image impedance for a two port network? State two problems
with constant-k filters and explain how an m-derived filter is able to remedy
these problems. Sketch the attenuation in the stopband for a constant-k filter
and an m-derived filter.
(16 marks)
(d) Design a constant-k low-pass filter with a cut-off frequency,
ωc=2π×3×109 Rad/s, and characteristic impedance, R0=k=50 Ω.
(8 marks)
PLEASE TURN OVER
3
A2 Answer ALL parts (a) – (d)
(a) With the aid of relevant diagrams explain the difference between Class A, B
and C amplifiers. Discuss the relative advantages and disadvantages of each.
(12 marks)
(b) Explain why GaN is preferred to Si for high power, single frequency
applications?
(10 marks)
(c) The following is an equation for the gain of a transistor-based RF amplifier
Where S11, S22 and S21 are the input reflection, output reflection and forward
transmission of the transistor and s and L are the reflections of the sources
and load matching circuits. State the requirements for the input and output
reflection on the matching circuits to achieve maximum unilateral transducer
gain. If the transistor has S11=0.2+j0.3, S22=0.1-j0.5, S12=0 and S21=12, alter the
equation above to find the maximum gain and attribute each term to sections
of the circuit diagram of a typical single stage amplifier.
(14 marks)
(d) What does the Bode-Fano criterion tell us about the relationship between
bandwidth and impedance match? Briefly describe two methods of improving
the bandwidth or match of an amplifier and explain what happens to the
gain-bandwidth product in each case.
(14 marks)
PAPER ENDS
= |21|
2 ∙
(1 − ||
2)
(|1 − 11|2)
∙
(1 − ||
2)
(|1 − 22|2)